In this world Everybody gets thirsty. The urge to drink
fluids is a natural instinct regulated by a negative feedback loop between the
brain and other organs in the body. In the elderly and others, however, that
loop sometimes weakens, putting their health dangerously at risk. By unraveling
the complexity of the thirst mechanism, scientists are developing better
treatments for people who lose their sense of thirst and are gaining greater
knowledge about many other basic human behaviors.
The positron emission tomography (PET) image at left,
taken after subjects received an infusion of a concentrated saline solution
into the blood to stimulate thirst, shows regions of activity in the left side
of the brain in thirsty subjects. This activity changed dramatically after
their thirst was quenched. In particular, the yellow and orange areas above
indicate activity along the cingulate cortex that was extinguished later.
Thirst. As far as survival instincts go, it doesn’t get
more basic. Water is the most abundant molecule in the human body, making up
about 70 percent of our body weight. It performs a host of important internal
functions, from maintaining body temperature; to transporting vitamins,
minerals, hormones, and other substances; to lubricating joints, eyes, and
intestines.
We can survive for only a matter of days without water.
Thirst serves as an automatic reminder of that fact—and, thus, plays a crucial
role in keeping us alive.
Yet people sometimes lose their sense of thirst. The
elderly in particular are prone to not feeling thirsty, even as they become
dehydrated. Certain brain injuries also can prevent people from recognizing
when they need to drink.
Recently, reports of people, particularly marathon
runners, who drink too much water in an overzealous attempt to avoid
dehydration have hit the news. The excess water overwhelms the kidneys, which
can’t flush out the fluid fast enough. The water then rushes into cells
throughout the body, swelling them like balloons. In the brain, such swelling
can be disastrous, causing seizures, coma, respiratory arrest, and death.
Recent research is helping scientists gain greater
knowledge about thirst and the brain’s role in regulating it. These studies are
leading to:
A better understanding of how aging mutes the sense of
thirst.
Greater insight into diabetes insipidus (“water”
diabetes) and other diseases for which excessive thirst is a symptom.
Safer recommendations for how much water marathon runners
and others engaged in endurance exercise should consume.
Scientists have been studying the neurological mechanisms
of thirst for decades. Early on, they discovered that the body’s primary
“thirst center” in the brain is the hypothalamus, a deep structure that also
regulates body temperature, sleep, and appetite. Special sensors in the
hypothalamus are constantly monitoring the blood’s concentration of sodium and
other substances. The hypothalamus also receives inputs from sensors in the
blood vessels that monitor blood volume and pressure. When blood volume or
pressure falls too low—from bleeding, for example, or from the excessive loss
of fluid in sweat or diarrhea, or when blood sodium concentration rises too
high from eating salty snacks, or as the result of certain diseases, the
hypothalamus sends out a strong message: Drink something. Now.
For reasons that aren’t clear, age tends to dampen this
message from the brain. In rare cases, when an aneurysm or other brain injury
has destroyed the sensors in the hypothalamus that regulate blood sodium
concentration, people can lose their sense of thirst completely. They must be
prescribed a fixed amount of fluids daily to keep their body safely hydrated.
Thirst isn’t the brain’s only response to dehydration.
When the body gets low on water, the hypothalamus increases the synthesis of an
antidiuretic hormone called vasopressin, which is secreted by the pituitary
gland and travels to the kidneys. There, it causes water to be reabsorbed from
the urine, thus reducing urine flow and conserving water in the body until more
fluids are consumed.
If the pituitary gland becomes damaged, however, or if
the kidneys are unable to respond to vasopressin, the body is unable to
conserve fluids. The result can be diabetes insipidus, a condition marked by
excessive urination and extreme, uncontrollable thirst. (Diabetes insipidus
should not to be confused with diabetes mellitus, which also causes excessive
thirst and urination, but which results from an insulin deficiency or
resistance that leads to high blood glucose.)
Until scientists understood the structure of vasopressin
and its role in diabetes insipidus, people with the condition had to drink up
to 20 quarts of water daily to stay healthy. Today, however, diabetes insipidus
can be successfully treated with the synthetic drug demopressin, which mimics the
action of vasopressin.
Recently, scientists have discovered that vasopressin
secretion increases (and, thus, less body fluid is lost) during periods of
physical stress. For that reason, many medical experts are now recommending
that healthy runners drink only when thirsty during marathons to avoid
retaining excess water with potentially dire consequences.
Although much has been learned about the neural
regulation of thirst, research continues. Scientists are exploring, for
example, why such factors as swallowing and the emptying of fluids from the
stomach appear to inhibit thirst even before the body becomes fully hydrated.
Studies into the thirst mechanism also are helping unravel some of the
mechanisms by which the brain motivates sleep, appetite, and other basic human
instincts.
Further
Reading
Almond CS, Shin AY, Fortescue EB, et al. N. Engl. J. Med.
2005;352:1550-1556. Hyponatremia among runners in the Boston Marathon.
Egan G, Silk T, Zamarripa F,et al. PNAS.
2003;100:15241-15246. Neural correlates of the emergence of consciousness of
thirst.
Greenberg A, Verbalis JG. Kidney Int. 2006;69:2124-2130.
Vasopressin receptor antagonists.
McKinley MJ, Johnson AK. News Physiol. Sci. 2004;19:1-6.
The physiological regulation of thirst and fluid intake.
Parsons LM, Denton D, Egan G, et al.
PNAS.2000;97:2332-2336. Neuroimaging evidence implicating cerebellum in support
of sensory/cognitive processes associated with thirst.
Stricker EM, Sved AF. Nutrition. 2000;16:821-826. Thirst.
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